1. Field of the Invention
The present invention relates to an electron-beam curable polyurethane resin for magnetic recording media (hereinafter referred to as an electron-beam curable resin in some cases), to a method for producing the same, and to a magnetic recording medium using the same. More specifically, the present invention relates to an electron-beam curable polyurethane resin having excellent crosslinking characteristics and which is suitably used for magnetic recording media, to a method for producing the electron-beam curable polyurethane resin having excellent crosslinking characteristics by electron-beam sensitive modification of a common thermosetting polyurethane resin, and to a magnetic recording medium using the electron-beam curable polyurethane resin.
2. Description of the Related Art
Resins conventionally used for magnetic recording media typically include thermosetting resins and electron-beam curable resins. The thermosetting resins are cured by allowing active hydrogen in the resins, which is typically the hydroxy group, to react with an isocyanate compound to form crosslinks in the resins. On the other hand, the electron-beam curable resins are cured by introducing an electron-beam sensitive functional group, which is typically the acrylic double bond, and exposing the resins to an electron beam to form crosslinks.
In general, vinyl chloride resins and polyurethane resins are used as the electron-beam curable resins for magnetic recording media. For electron-beam sensitive modification of the vinyl chloride resins, the hydroxy group of a thermosetting vinyl chloride resin having a hydroxy group may be allowed to react with a tolylene diisocyanate (TDI) adduct produced by a reaction between TDI and 2-hydroxyethyl methacrylate (2-HEMA) (disclosed in Japanese Examined Patent Application Publication No. 1-25141), allowed to react with a cyclic anhydride and further react with an epoxy monomer having an acrylic double bond (disclosed in Japanese Patent No. 2514682), or allowed to react with 2-isocyanateethyl(meth)acrylate (MOI) (disclosed in Japanese Unexamined Patent Application Publication NO. 4-67314).
On the other hand, in the case of using the polyurethane resins, typically, a (meth)acrylate compound having a hydroxy group in the molecule thereof may be used as part of the material for synthesizing polyurethane to produce a radiation curable polyurethane resin (disclosed in Japanese Patent No. 2610468), or a polyurethane whose polymer end is a isocyanate group may be prepared and subsequently allowed to react with an alcohol having an acrylic double bond (disclosed in Japanese Examined Patent Application Publication No. 3-1727).
As for the electron-beam curable polyurethane resins, if an acrylic double bond can be introduced to the active hydrogen, which is typically the hydroxy group, of a known thermosetting polyurethane resin, in the same manner as in the vinyl chloride resins, commercially available polyurethane resins can be modified to be electron-beam sensitive.
The thermosetting polyurethane resin however has a smaller amount of active hydrogen in the molecule thereof than that of thermosetting vinyl chloride resins. Hence, when the modification is performed by the same technique as in the vinyl chloride resins, only a small number of acrylic double bonds can be introduced and, consequently, crosslinking of the coating film cured by electron-beam is disadvantageously insufficient.